1. Field of the Invention
The present invention relates to a solid state imaging device and, more particularly, to a solid state imaging device using a floating diffusion amplifier as the charge detector for detecting a charge signal transferred from a horizontal charge transferring unit.
2. Description of the Prior Art
FIG. 3 is a block diagram of a CCD solid state imaging device that embodies the invention by operating as an interline transfer system. In FIG. 3, the device has an imaging region 4 made of a plurality of photo-sensors 1, a plurality of read gates 2 and a plurality of vertical CCD's (vertical charge transferring means) 3. The photo-sensors constitute a two-dimensional picture element-based sensor array that accumulates a charge signal proportionate to the amount of the light received. The charge signal is read out instantaneously during a vertical blanking period via the read gates 2. The charge signal thus read out is transferred vertically by the vertical CCD's 3.
On the output side of the imaging region 4 is a horizontal CCD (horizontal charge transferring means) 5. Receiving the charge signal transferred per line from the vertical CCD's of the imaging region 4, the horizontal CCD 5 transfers the signal horizontally in accordance with the horizontal scanning period of a TV signal. After leaving the horizontal CCD 5, the charge signal is detected by a charge detector 6 that produces a corresponding electrical signal.
Prior art CCD solid state imaging devices of the above-described type are designed to have a horizontal CCD channel region 10 wide enough to handle adequately the charges in the horizontal CCD 5, as illustrated in FIG. 4. At the same time, this type of CCD solid state imaging device includes the smallest possible FD (floating diffusion) region 7 of the floating diffusion amplifier implemented by the charge detector 6. The FD region 7 thus restricted is intended to enhance the sensitivity of the CCD solid state imaging device.
As shown in FIG. 4, the prior art CCD solid state imaging device lets the charge signal spread fully across the width of the horizontal CCD channel region 10 and then throttles the signal abruptly at a horizontal output gate (HOG) 8 of the horizontal CCD 5 for charge transfer to the FD region 7. Because designing the FD region 7 smaller necessarily narrows the channel width W of the outlet of the horizontal output gate 8, there occurs what is known as the narrow channel effect.
FIG. 5 is a section taken on line A--A'in FIG. 4. The narrow channel effect produces a shallow potential region at the outlet of the horizontal output gate 8, the region being enclosed by broken line P in FIG. 5. The region forms a potential barrier that traps, in the horizontal output gate 8, part of the charge signal transferred thereto from the horizontal CCD 5. The trapped charge is not transferred to the FD region 7.
Making the FD region 7 smaller makes the outlet of the horizontal output gate 8 narrower in channel width W. The resulting narrow channel effect produces the potential barrier at the outlet of the horizontal output gate 8, the barrier degrading the efficiency of charge signal transfer. Conversely, the predicted deterioration in charge signal transfer at the horizontal output gate 8 is a prior art impediment that discourages making the FD region 7 smaller to enhance the sensitivity of the CCD solid stage imaging device.